This invention relates generally to carbon nanotubes and nanofibers, and more particularly to methods for manipulating the length and functionality of carbon nanotube and nanofiber materials, preferably without sidewall damage.
Take single-walled carbon nanotubes (SWNTs) as an example. SWNTs are reported to have a high elastic modulus in the range of 500-600 GPa, with a tensile elastic strength of about 200 GPa. SWNTs are considered among the most promising reinforcement materials for the next generation of high-performance structural and multifunctional composites, as well as nanotube-based device applications. However, developing these SWNT-based materials and devices poses major challenges. Their large surface area, high van der Waals attraction, and large aspect ratios cause SWNTs to bond together and form long bundles and ropes. This specific phenomenon is called nanoscale self-assembly, which creates problems in nanotube dispersion and tube-to-tube sliding under tension. The interfacial bonding between the SWNTs and the matrix is usually poor due to the nanotubes' structure perfection and high chemical stability. This also causes load transfer problems, such that SWNTs will pull out from the matrix in nanocomposites instead of fracturing.
In addition, the SWNT hollow structure cannot be effectively used since conventional nanotubes are very long (relative to their diameter) and capped with fullerene structures. Nanotubes should prove to be good molecule containers for various novel applications, such as storage of hydrogen and drugs, as well as enhancing interfacial bonding with other materials at open ends, if they can be properly uncapped and functionalized at open ends.
Opened nanotubes are highly desired for the development of high performance materials and devices, for both scientific research and industrial applications. Accordingly, a number of techniques have been developed to cut nanotubes. Chopping nanotubes to create open ends in the tubes is an effective technical solution to enhance tube dispersion and interfacial bonding in nanocomposites. However, directly cutting or chopping nanotube materials is extremely difficult, due in part to the small size of nanotubes (typically several nanometers in diameter and less than one micron in length).
Current methods for shortening or cutting nanotubes can be classified into two basic categories: chemical methods and physical methods. Most chemical methods use highly concentrated acid to treat the nanotubes, potentially seriously damaging or destroying the nanotubes' unique molecular structure, which consequently would lead to significant degradation or loss of mechanical and functional nanotube properties. On the other hand, conventional physical cutting methods are time-consuming and difficult to control, requiring expensive equipment, while offering severely limited processing capacity. For example, with grinding and milling (e.g., with ceramic balls) techniques, it is difficult to control the length of the nanotubes and the process is time-consuming. Sonication cutting methods may damage the nanotubes' sidewalls. As still a further example, cutting with the aid of expensive high-resolution microscopes, such a scanning electronic microscope (SEM), an atomic force microscope (AFM), or with a lithography technique, would allow better control of the nanotube length, but only a few tubes or tube ropes could be processed at a time.
Accordingly, it would be desirable to provide improved methods for cutting or chopping carbon nanotubes, so as to effectively and efficiently control the length and functionality of the carbon nanotubes and nanofiber materials while minimizing sidewall damage. It would also be desirable to fill chopped nanotubes and enhance nanotube dispersion and interfacial bonding for producing high performance nanocomposites and device applications.